1. Field of the Invention
The present invention relates to a semiconductor device provided with a function of controlling current supply to a load by using a transistor, and more particularly to a semiconductor device which includes a pixel composed of a current drive light-emitting element whose brightness varies with a current and a signal driver circuit for driving the pixel, and a driving method thereof.
2. Description of the Related Art
Recently, a display device whose pixel is composed of a light-emitting element such as a light emitting diode (LED), that is a self-light emitting display device has been in the spotlight. Among light emitting elements used for the self-light emitting display device like the above, an organic light emitting diode (OLED), an organic EL element, and an electroluminescence (EL element) have been drawing attention and been more likely to be used for an organic EL display.
Since such a light-emitting element emits light by itself, it enables higher pixel visibility as compared to a liquid crystal display and does not require a backlight. Further, it exhibits high response speed and the brightness of the light-emitting element can be controlled corresponding to the current value flowing in the light-emitting element.
A passive matrix drive and an active matrix drive are known as the driving method of a display device using a self-light emitting element. Although the passive matrix drive has a simple configuration, there are problems such as the difficulty in realizing a display with large size and high brightness. Therefore, the active matrix drive in which a current flowing in a light-emitting element is controlled by a thin film transistor (TFT) which is disposed in a pixel circuit has been developed actively.
An active matrix display device has problems in that a current flowing in a light-emitting element varies due to variations in current characteristics of a driving TFT and thus brightness of each light-emitting element which structures a display screen varies. That is, the active matrix display device has a driving TFT for driving a light-emitting element into which a current flows in a pixel circuit and the current flowing in the light-emitting element varies due to the characteristic variations of the driving TFT, thus brightness varies.
In view of the aforementioned problems, various circuits are proposed in order that no current flowing in a light-emitting element varies even when characteristics of a driving TFT in a pixel circuit vary and thus variations of brightness is suppressed (e.g., see Patent Documents 1 to 4).
[Patent Document 1]
Japanese Patent Unexamined Publication No. 2002-517806
[Patent Document 2]
International Publication No. 01/06484 pamphlet
[Patent Document 3]
Japanese Patent Unexamined Publication No. 2002-514320
[Patent Document 4]
International Publication No. 02/39420 pamphlet
Configurations of active matrix display devices are disclosed in Patent Documents 1 to 4 and disclosed particularly in Patent Documents 1 to 3 are circuit configurations in which no current flowing in a light-emitting element varies due to characteristic variations of a driving TFT in a pixel circuit. This configuration is referred to as a current write type pixel or a current input type pixel. Disclosed in Patent Document 4 is a circuit configuration for suppressing variations in a signal current due to variations of a TFT in a source driver circuit.
A configuration of a conventional active matrix display device disclosed in Patent Document 1 is shown in FIG. 6. A pixel in FIG. 6 includes a source signal line 601, first to third gate signal lines 602 to 604, a current supply line 605, TFTs 606 to 609, a storage capacitor 610, an EL element 611, and a current source for inputting a video signal 612.
A gate electrode of the TFT 606 is connected to the first gate signal line 602, a first electrode thereof is connected to the source signal line 601, and a second electrode thereof is connected to first electrodes of the TFTs 607, 608, and 609. A gate electrode of the TFT 607 is connected to the second gate signal line 603 and a second electrode thereof is connected to a gate electrode of the TFT 608. A second electrode of the TFT 608 is connected to the current supply line 605. A gate electrode of the TFT 609 is connected to the third gate signal line 604 and a second electrode thereof is connected to an anode of the EL element 611. The storage capacitor 610 is connected between the gate electrode of the TFT 608 and the current supply line 605 so as to store a voltage between the gate and the source of the TFT 608. The current supply line 605 and a cathode of the EL element 611 are respectively input predetermined potentials to have a potential difference therebetween.
Operation through a signal current writing to a light emission will be explained using FIGS. 7A to 7E. Reference numerals denoting respective parts conform to those shown in FIG. 6. FIGS. 7A to 7C schematically show current flows. FIG. 7D shows the relationship between currents flowing in respective paths when a signal current is written. FIG. 7E shows a voltage stored in the storage capacitor 610, namely a voltage between the gate and the source of the TFT 608 when a signal current is written.
Firstly, a pulse is input to the first gate signal line 602 and the second gate signal line 603 and the TFTs 606 and 607 are turned ON. Current flowing through the source signal line 601, namely a signal current is referred to as Idata here.
As shown in FIG. 7A, since the signal current Idata is flowing through the source signal line 601, the current flows separately through current paths I1 and I2 in the pixel. FIG. 7D shows the relationship between the currents. Needless to say, the relationship is expressed as Idata=I1+I2.
Charge is not yet stored in the storage capacitor 610 at the instant when the TFT 606 is turned ON, and therefore the TFT 608 is OFF. Consequently, I2=0 and Idata=I1. That is, only a current flows due to the accumulation of charge in the storage capacitor 610 during this period.
The charge is then accumulated gradually in the storage capacitor 610, and a potential difference starts to generate between both electrodes of the storage capacitor 610 (see FIG. 7E). The TFT 608 is turned ON when the potential difference between both electrodes reaches Vth (point A in FIG. 7E), and I2 is generated. Since Idata=I1+I2 as mentioned above, I1 is gradually reduced. However, current still flows and charge is further accumulated in the storage capacitor 610.
The charge continues to be accumulated in the storage capacitor 610 until the potential difference between both electrodes of the storage capacitor 610, namely the voltage between the gate and the source of the TFT 608 reaches a desired voltage that is a voltage (VGS) at which the TFT 608 can flow the current Idata. When the accumulation of charge is complete (point B in FIG. 7E), the current I1 stops flowing and a current corresponding to VGS at this time starts to flow into the TFT 608, thus satisfies Idata=I2 (see FIG. 7B). In this way, the steady state is obtained. Signal writing operation is thus complete. When the selection of the first gate signal line 602 and the second gate signal line 603 is completed, the TFTs 606 and 607 are turned OFF.
In the subsequent light emitting operation, a pulse is input to the third gate signal line 604 and the TFT 609 is turned ON. Since the previously written VGS is stored in the storage capacitor 610, the TFT 608 is ON and the current Idata flows from the current supply line 605. Therefore, the EL element 611 emits light. In the case where the TFT 608 can operate in a saturation region, Idata can continue to flow without changing at this point even if the voltage between the source and the drain of the TFT 608 changes.
The operation for outputting a set current as described above is referred to as an output operation here. The current write type pixel as described above has an advantage in that a desired current can be supplied to an EL element accurately and thus variations in brightness due to the characteristic variations of TFTs can be suppressed since a voltage between the gate and the source which is required for flowing the current Idata is stored in the storage capacitor 610 even when the TFT 608 has characteristic variations and the like.
The above-described example relates to a technique for correcting current variations due to variations in driving TFT's in a pixel circuit; however, the same problem arises in a source driver circuit. A circuit configuration for preventing variations in signal currents due to manufacturing variations of TFTs in a source driver circuit is disclosed in Patent Document 4.
In this manner, a conventional current drive circuit and a display device employing it are configured so that the relationship between a signal current and a current for driving a TFT, or the relationship between a signal current and a current flowing in a light-emitting element during light emission can be equal or stay in proportion to each other.
In the cases where a driving current of a driving TFT for driving a light-emitting element is small or where a dark gradation is to be displayed by a light-emitting element, the signal current decreases accordingly. Since parasitic capacitance of a wiring used for supplying a signal current to a driving TFT and a light-emitting element is quite large, a time constant for charging the parasitic capacitance of the wiring becomes large when the signal current is small, which makes the signal writing speed slowed down. That is, the speed for supplying a current to a transistor, thereby generating a voltage at the gate terminal of the transistor becomes slow, which is required for the transistor to flow the current.
In view of the foregoing problems, technologies for improving the signal writing speed have been studied (e.g., see Patent Documents 5 and 6).
[Patent Document 5]
Japanese Patent Examined Publication Number 2003-50564
[Patent Document 6]
Japanese Patent Examined Publication Number 2003-76327
A display device provided with a current control means by which a data line current supplied by a data line drive means is divided into a data current for writing brightness information to each of pixel circuits and a bypass current to drive is disclosed in Patent Document 5. For example, as shown in FIG. 33, a pixel circuit in which no brightness data is written is used as a data current control circuit (bypass current).
The drive timing is shown in FIGS. 34 and 35. Sequential x pixel circuits (x=2 in FIG. 33) are selected at the same time. When two pixel circuits are selected at the same time, a part of a data line current for driving a data line is written into one pixel circuit as a brightness data current. To a part of the other pixel circuit, the brightness data current is not written, however, it is used as a data current control circuit (bypass current) to which the rest of the data line current flows.
In particular, in FIG. 35, sequential x pixel circuits (x=2 in FIG. 33) in the same column are sectioned as one block. When a data current is written to one pixel circuit within this block, no data current is written to the other pixel circuit within the block and the pixel circuit is used as a bypass current. At this time, in the pixel circuit to which a data current is written, both a first scan line WS and a second scan line ES are selected. In FIG. 33, assuming that a pixel circuit 11-k-1 is a pixel circuit for writing a data current for example, both WSk-1 and ESk-1 are selected.
On the other hand, in a pixel circuit to which no data current is written and used as a bypass current, only a first scan line WS is selected. In FIG. 33, WSk is selected and a second scan line ESk is not selected. Consequently, the pixel circuit functions as a data current control circuit in which TFTs 24 and 25 are used as bypass currents. That is, in the pixel circuit shown in FIG. 33, since the second scan line ESk is not selected and a TFT 26 is OFF, charge corresponding to brightness data which is stored in the capacitor 23 is prevented from being discharged through the TFT 26 and remains stored. At that time, a part of the circuit, namely only the TFTs 24 and 25 function as data current control circuits (bypass current).
As described above, in an active matrix organic EL display device using a current write type pixel circuit, sequential two pixel circuits in the same column are selected at the same time and a part of a data line current Iw0 is supplied to a pixel circuit to be written brightness data while the rest of the current is supplied to a part of the other pixel circuit as a bypass current. As a result, it is possible to set the data line current Iw0 larger than a data current Iw1 flowing in the TFTs 24 and 25 while suppressing the size of the TFT's 24 and 25 in the pixel circuit. Therefore, it becomes possible to drastically reduce the data writing time, thus contributes to the realization of an organic EL display device with larger size and higher definition.
A circuit shown in FIG. 36 is disclosed in Patent Document 6. That is, a driving transistor 7 is connected in parallel to an auxiliary transistor 12 having a current drive capacity of n times as large as that of the driving transistor 7 so that a drain current flows also to the auxiliary transistor 12 and a signal current flowing through a signal line 3 becomes (n+1) times as large during a part of the selection period (acceleration period). Therefore, charge and discharge of the storage capacitor and the parasitic capacitor can be performed at fast speed and a gate potential of the driving transistor reaches a predetermined potential during the selection period without failing, thus a current driving element can be driven with an appropriate driving current even in the case of a small signal current (input signal). Therefore, when a current driving element is an organic EL element, the organic EL element can be driven with a predetermined driving current and thus deterioration of display image quality is prevented.